This invention relates to lubricants for the processing of thermoplastic resins and to a process for shaping thermoplastic synthetic resins. More particularly, this invention relates to lubricants which are useful at high temperatures for the processing of thermoplastic synthetic resins and to a high temperature process for shaping thermoplastic synthetic resins.
Processes for shaping thermoplastic synthetic resins are well known, such as calendering, pressing, injection molding, extruding, and the like, at elevated temperatures and under compression loads. The heated mass of plastic material in such processing tends to adhere to the heated parts of the machine. The adhering material reaches a substantially longer retention time in the machines, resulting in the potential for greater thermal degradation. The foregoing results in the exhaustion of the stabilizer which is intended to prevent such degradation, although the processing operation has not yet been terminated. This results in the premature termination of the processing procedure.
In order to avoid the foregoing difficulties, suitable lubricants are added to the plastic powder in addition to the stabilizers, which lubricants are intended to reduce the internal and external friction of the melted mass and in such a manner to reduce the effect of shearing forces and/or to diffuse between the material in the plastic state and the heated parts of the machines, in order to prevent adhesion, resulting in longer retention of the material to be processed.
Good lubricants are already known in the art such as various types of waxes, fatty alcohols, glycerides, fatty acid amiles, fatty acid esters with low alcohols, such as butyl stearate, fatty acids, and paraffins.
.alpha.olefin maleic copolymers and their derivatives have already been proposed as processing aids for use in masses of thermoplastic synthetic resins. From German Patent Disclosure No. 2,015,162 it is know to provide thermoplastic molding batches of polyester for injection molding purposes with a coating of ionic copolymers of .alpha.-olefins and salts of .alpha., .beta.-unsaturated monocarboxylic acids or dicarboxylic acids, a result of which is that the plastic material, after it has been molded, can easily be removed from the molds.
From British Pat. No. 764,014 it is known to add at least 0.1%, by weight, of a copolymer of an unsaturated acid anhydride with styrene, an olefin, or an alkylvinyl ether with alkyl groups of 1 to 25 carbon atoms to polyvinylchloride (PVC). The addition of this modifier greatly increass the impact tenacity. For this purpose, the copolymers are added in a quantity from 0.1 to 5%, by weight, and, during rolling are resistant at temperatures between 149.degree. and 180.degree. C., without addition of a special lubricant to the PVC powder. At higher temperatures, however, the lubricating effect declines to a greater extent, so that in the instance of this copolymer, a rolling resistance sufficient for processing does not exist.
Pursuant to an earlier proposal, esterified .alpha.-olefin/maleic acid copolymers were discovered to be universal lubricants for the shaping of plastics; they act as internal, as well as external, lubricants.
It should be noted, however, that the foregoing lubricants which have been proposed so far in the art develop an adequate lubricating effect only at the customary processing temperatures, as they are, for example, usually employed in all extrusion processes. At higher temperatures, the lubricating effect appears to decline very rapidly.
In some processing methods, and when certain stabilizer systems are employed, it is necessary to perform the processing at temperatures from about 190.degree. to about 210.degree. ., in order to obtain optimal mechanical properties of the shaped parts. For example, the flowability of PVC required for injection molding lies in a temperature range of about 200.degree. C., at which temperature the polymer begins to display a large tendency to become degraded. The polymer must therefore be plasticized quickly, before degradation occurs. In the calendering process, in particular, high temperatures are reached because of the high shearing forces. Furthermore, the relatively extensive exposure to atmospheric oxygen will result in a decline in stability during processing on a calender. In a calender system, a temperature of about 200.degree. C., or greater, is reached in the last roller gap. Consequently, in PVC calendering it is necessary to adapt the selection of stabilizer systems and processing auxiliaries to these high temperatures.
For manufacturing film for food packaging it is necessary to replace the toxic Ba-Cd stabilizers by organo-tin compounds. Customarily, mixtures of solid and of liquid tin stabilizers are employed. The foregoing is being done because of the difficulties associated with distributing the solid tin stabilizers. However, plastic masses containing liquid tin stabilizers exhibit a great adhesion tendency at processing temperatures of about 200.degree. C. Such a tendency cannot be prevented by using the known lubricants and requires special flowing aids, such as acrylates. Even when formulations with very high proportions of tin stabilizers having a low adhesion tendency are employed, it is not possible to dispense with flowing aids. In addition to such flowing aids, high molecular polyethylene waxes and solid parafins are employed to prevent adhesion. It should be noted, however, that the addition of such agents only solves the problem of adhesion during the processing operation. In order to prevent adhesion of the film in the roll, or the adhesion of the shaped parts in the mold, in the case of injection molding, it is necessary to us amide waxes which have especially high release effects, as anti-blocking agents, in addition to the flowing aids.
A great disadvantage results from the use of such flowing and anti-blocking auxiliaries in as much as their use limits the possibility for modifying the formulations. Furthermore, the typically employed amide waxes leads to deterioration in stability and color. Such difficulties occur, in particular, with stabilizer combinations containing liquid alkyl tin maleates and such stabilizers are used frequently, because of their excellent light stability.
In view of the foregoing, it was an object of the present invention to develop lubricants which, in the shaping of thermoplastic resins, act simultaneously as lubricants and parting compounds, resulting in a parting effect which is equal to, or in excess of, the known anti-blocking effects of the amide waxes, and which display an acceptably high lubricating effct, even at extreme processing temperatures.